1. Technical Field
The present invention relates to an apparatus and a method for controlling a vehicle.
2. Related Art
Japanese Unexamined Patent Application Publication (JP-A) No. 2009-126186, for example, discloses a technique, adopted in an electric power steering apparatus of a vehicle, of performing a correction of increasing a steering angle characteristic when the steering frequency is in a range lower than or equal to a predetermined value and of reducing the steering angle characteristic when the steering frequency is in a range higher than or equal to the predetermined value, to prevent a deterioration in a steering feeling while reducing a deterioration in vehicle response.
In addition, JP-A No. 2013-103664 discloses a technique, adopted in an electric power steering control apparatus of a vehicle, of correcting the amount of assistance such that a gain characteristic for an assist correction amount is gradually decreased if the frequency of steering wheel torque exceeds a first set value, and the gain characteristic for the assist correction amount is gradually increased if the frequency of an estimated road surface reaction force exceeds a second set value, to reduce rolling vibrations while preventing a reduction in yaw response of the vehicle.
Further, JP-A No. 2014-141174 describes control of applying a control target torque, which amounts to a basic assist torque minus a torque corresponding to viscosity in steering, in electric power steering (EPS) control for lane keep assist, to achieve an improved steering feeling and reduced driver fatigue. JP-A No. 2014-141174 describes a function of increasing a viscous torque as a yaw rate difference approaches 0, a function of decreasing the viscous torque as a vehicle speed increases, and a function of calculating the viscous torque in accordance with an angular velocity of a steering wheel.
Furthermore, JP-A No. 2012-210832 describes a technique of performing braking/driving force control using a difference between a target stability factor and an actual stability factor.
However, the technique described in JP-A No. 2009-126186 mentioned above has a problem in that, as the frequency related to steering increases, the amount of control on the steering decreases, which results in a reduction in turning performance of the vehicle. This may cause a discrepancy between the steering by a driver and a turn of the vehicle, which may lead to a failure to cause the vehicle to make a turn along a driving course as intended by the driver. Accordingly, there has been a demand for a technique for compensating for reduced turning performance of the vehicle in accordance with an increase in the frequency related to the steering.
Further, according to the technique described in JP-A No. 2014-141174 mentioned above, the viscous torque is increased when the vehicle is traveling straight ahead, and when control on the steering is lost because of an uneven road surface when the vehicle is traveling on a rough road, a self aligning torque which may be caused by the loss of the control on the steering can be easily reduced. However, when the vehicle is traveling straight ahead at a high speed, the viscous torque is reduced, and if a loss of control on the steering is caused by an uneven road surface, and a self aligning torque that is so great as to cancel lane keep assist control occurs, the control may be terminated at a time at which the driver does not expect the control to be finished, which may lead to an uncomfortable feeling of the driver and unstable behavior of the vehicle. Further, because the control is performed with the steering alone, a response characteristic of the vehicle with respect to the amount of a steering operation cannot be changed, and the response characteristic of the vehicle with respect to the steering (i.e., the phase of a yaw with respect to the steering) cannot be controlled. Furthermore, turn assist control or control to improve stability performance cannot be selectively performed depending on whether the road surface on which the vehicle is traveling is in a high μ condition or a low μ condition. Furthermore, when the steering wheel is operated when the vehicle is traveling at a low speed on a low μ road surface, the behavior of the vehicle can be easily stabilized because of an increased viscous torque, but when the steering wheel is operated when the vehicle is traveling at a high speed on a low μ road surface, the reduced viscous torque may cause an unintended excessive turn of the vehicle and a reduction in the stability performance of the vehicle.
Furthermore, according to the technique described in JP-A No. 2012-210832 mentioned above, a control amount is calculated based on the target value and actual value (i.e., calculated value) of the stability factor for motor driving, and therefore, the function that can be accomplished by this technique is similar to that of known yaw rate feedback control due to limitations of vehicle motion control, and this technique does not allow a characteristic (i.e., a target value) of the stability factor to change in accordance with the driving condition. Therefore, only one of turn assist and stability performance of the vehicle can be emphasized, and control to reduce shaky steerings is not performed, and therefore, when corrections in steering are repetitively performed on a low μ road surface, for example, a reduction in the stability performance of the vehicle may occur.